Cathode arrangement of an electron microscope for reducing the occurrence of virtualcathodes



1968 J. w. COLEMAN ETAL ,363, 61

CATHODE ARRANGEMENT OF AN ELECTRON MICROSCOPE FOR REDUCING THE OCCURRENCE OF VIRTUAL CATHODES Filed Oct. 22, 1965 lnveniors;

Patented Jan. 16, 1968 CATHODE ARRANGEMENT OF AN ELECTRON MICROSCOPE FOR REDUCING THE OCCUR- RENCE F VIRTUAL CATHODES John W. Coleman, Willingboro, and Joseph J. Schiller, Gibbsboro, N.J., assignors to Radio Corporation of America, a corporation of Delaware Filed Oct. 22, 1965, Ser. No. 500,803 7 Claims. (Cl. 313-82) ABSTRACT OF THE DISCLOSURE In known electron guns, where the cathode is substantially surrounded by a focusing grid comprising a unipotential surface, standing waves occur inside the grid. Since electrons collect at the nodes of the standing waves, whereby the nodes of the standing waves act as virtual cathodes, the electron beam flowing out of the grid will compiise electrons from the cathode and also electrons from the virtual cathodes. The electrons from the virtual cathodes cannot be focussed at the same point as the electrons from the cathode, whereby the resultant electron beam cannot be pro erly focussed. Means are provided to prevent formation of virtual cathodes in a region so near to the cathode as to contribute electrons to the electron beam, whereby the cathode ray beam is focusable.

This invention relates to a cathode arrangement which produces a focusable beam for use in beam devices such as an electron microscope.

In electron microscopes, a focusable beam must be provided; to this end, a point type of electron emitter or cathode may be provided, since, where all the electrons comprising a beam take the same path at the same velocity, the electron lenses through which the electrons pass produce the same focusing action on all the electrons. However, in a point source type of cathode, which includes a collimating grid surrounding the cathode, standing waves at very high frequency appear within the hollow grid and in proximity to the cathode. At each node of the standing wave, electrons collect, forming virtual cathodes, which act as undesired apparent sources of electrons. The beams of electrons from these virtual cathodes, which are spaced from the point type of cathode, will not be focused at the same point as the electrons from the point source by the electron lenses which focus the beam of electrons. Many superimposed images will be produced, one for the point source and one for each virtual source. These images will be laterally displaced whereby the overall effect is to blur the image. Those virtual cathodes which are so close to the point source that the electrons therefrom become part of the beam of electrons of the electron microscope and pass through the specimen have harmful effects, other virtual cathodes at greater distance from the point source not producing harmful effects. Therefore, the presence of standing Wave nodes near a point type cathode of an electron microscope degrades the image produced by the electron microscope.

It is an object of this invention to provide a point source of electrons that includes apparatus for preventing occurrence of harmful nodes of standing waves in the vicinity of the point source.

When standing waves exist, a whole number of half waves of the standing waves must extend between two points on a unipotential surface. In a point source type of an electron emitter, the grid, which substantially surrounds the point type cathode is a unipotential surface. Therefore, standing waves may extend between two inner points on the equipotential surface provided by the grid,

and each node of those standing waves is a virtual cathode. One or more of these virtual cathodes may be close enough to the point type of cathode as to provide electrons that pass through the specimen. According to the invention, means are provided to either prevent standing waves within the grid, or at least, to prevent the formation of standing waves which are in close enough proximity to the point source cathode as to cause degradation of the image. This means may comprise a floating electrode positioned within the grid. The floating electrode, being insulated from any other electrode, develops different charge distributions or voltages along the length thereof due to the proximity of parts of the floating electrode to other electrodes, such as the cathode and the leads thereto and the grid, which have different potentials. The floating electrode therefore presents different charge distribution along the length thereof and also does not present a surface having the same potential as any of the adjacent surfaces.

Therefore standing waves can neither start nor terminate on the floating electrode nor can they start or terminate on the floating electrode and terminate or start on an adjacent surface. The floating electrode either prevents standing waves from occurring within the grid or it so changes the pattern of standing waves that no node is produced near enough to the cathode as to contribute to the electron beam of the microscope that passes through the specimen, whereby degradation of the image by the action of one or more virtual cathodes is prevented. The floating electrode may take many shapes including the shape of a short rod supported on and insulated from the grid and extending towards the cathode, or the shape of a hollow electrode supported on and insulated from the grid surrounding at least part of the cathode.

The invention is more fully described in the following description when taken with the accompanying drawing in which:

FIGURES l and 2 are each a sectional view of a cathode arrangement for an electron microscope, each including a floating electrode for preventing the formation of harmful virtual cathodes.

Referring first to FIGURE 1, a cathode lit comprises a conical pin 12 of electron emitting material, such as tungsten, which is welded to the bent portion of a V- shaped heater wire 14, which may also be of tungsten. The legs of the heater wire 14 are welded to the bases 16 of two L-shaped leads 18. The leads 18 are so arranged that their legs 20 are parallel and so that their bases 16 are in line and extend towards each other. A focusing grid 22 substantially surrounds the pin 12, the heater wire 14 and a portion of each of the leads 18. The focusing grid 22 is of frusto-conical shaped conductive material having thin walls and two bases 24 and 26. The larger base 24 has two holes through which the legs 20' of the leads 18 extend. Buttons 28 support the leads 18 on the base 24 and insulate the leads 18 from the base 24. A small hole 30 is provided in the smaller base 26 in line with the pin 12 so that electrons produced by the cathode 10 may pass out of the grid 22. An anode 32, comprising a plate with a hole 34 therethrough is spaced from and is positioned parallel to the smaller base 26. An evacuated housing (not shown) surrounds and positions the grid 22 and anode 32.

In the operation of this device so far described, the anode 32 is grounded, and a high negative potential with respect to ground is applied to the cathode 10, while a higher negative potential with respect to ground is applied to the grid 22. Also, heating currents are passed through the heater 14 by way of the leads 18. The electrons produced by the cathode 10 are formed into a beam indicated by the dotted line 36 by the grid 22.

The grid 22 is an equipotential surface, whereby (in the absence of the floating electrode 38 to be described) standing waves may extend from one point within and on the grid 22 to another point within and on the grid 22. These standing waves are always of a certain high frequency, whereby two or more standing half waves may appear between two spaced points on the inside of the grid 22, the length of the path taken by the standing waves being an integral multiple of the length of a wave. At each node of the standing wave, electrons collect whereby one or more virtual cathodes are produced by the standing waves at the nodes thereof. If one of these virtual cathodes is in a position where the electrons therefrom are included in the beam that passes through the specimen (not shown), which is below the anode, that is, if one or more of the virtual cathodes is seen by the specimen, the electrons from the virtual cathode will not be focused at the same place as the electrons from the pin 12 by the lens (not shown) beneath the anode that focuses the beam from the point source, since the electrons from the virtual cathode come from a different position than the electrons from the point source comprising the pin 12. Therefore, the image produced by the electron microscope is degraded by the existence of these virtual cathodes. According to this invention, the floating electrode 38 is provided to prevent occurrence of harmful virtual cathodes.

In the embodiment of FIGURE 1, the rod-like floating electrode 38 takes the shape of a cone 40 having a rounded point 42 extending from a cylinder 44, the axes of the cone 4t and the cylinder 44 being in line. The floating electrode 38 is insulated from the grid 22 by an insulating ring .6 which fits around the free end of the cylinder 4-4. The insulator 45 has a flange 48 extending from one end thereof and the insulator 46 fits into a hole 49 in the base 24 which is midway between leads 18, the flange 48 being in contact with the top (as viewed in FIGURE 1) or" the base 24. Different charge distributions are induced in the floating electrode 38 along the length thereof due to the proximity of different portions thereof to the base 24 and to the leads 18 and the heater 14 and the pin 12. The base 24 on one hand is at a different potential from the leads 1?; and the heater 14 and the pin 12 on the other hand. Therefore, standing waves cannot extend between any two points on the electrode 38. Also, due to its position and to the potentials appearing along the length of the electrode 38, the path of standing waves extending between points on the inside of the grid 22 is so changed that either no standing waves (or virtual cathodes) appear, or those that do appear produce virtual cathodes that are not seen by the specimen.

In FIGURE 2, similar parts are given similar reference characters to those used in FIGURE 1. Due to the similarity of the two figures, no general explanation of FIG- URE 2 appears necessary. In FIGURE 2, the floating electrode 50 differs from the floating electrode 38 of FIG- URE 1 in that the floating electrode i comprises a hollow frusto-conical element having no base and which is positioned to surround part of the cathode and of the leads 18. The floating electrode 50 is supported on the base 24 by three legs 52 forming part of the electrode 50. Only two of these legs are shown in FIGURE 2. Three insulating buttons 54, one for each leg 52, support the floating electrode on the base 24 and insulate it therefrom. The floating electrode 50 of FIGURE 2 operates like the floating electrode 38 of FIGURE 1 in that different charge distributions appear along different portions thereof due to its proximity to the grid 22 and to the cathode 10, whereby no standing waves can originate or terminate on the floating electrode 50. Also by its presence, virtual cathode are prevented from forming where they could be seen by the specimen (not shown) below the cathode 10.

As is evident, other shapes and positions of floating electrodes are contemplated .by the described invention, therefore, the above description is merely illustrative.

What is claimed is:

ll. Means for preventing the occurrence of harmful virtual cathodes in a cathode-grid combination for an elec tron beam device in which said grid substantially surrounds and is insulated from said cathode, there being a hole in said grid through which electrons produced by said cathode may pass, said means comprising a floating electrode within said grid and insulating means to support said floating electrode in proximity to said cathode.

2. The invention as described in claim 1 wherein said floating electrode is rod-like and extends from a wall of said grid towards said hole therein.

3. The invention as described in claim 1, wherein said floating electrode comprises a hollow cylinder supported by said grid and surrounds a part of said cathode.

4. Means for preventing the occurrence of harmful virtual cathodes in a cathode-grid combination for an electron beam device in which said grid substantially surrounds said cathode and in which said cathode comprises a heater having legs, there being a hole through one wall of said grid through which electrons produced by said cathode pass, said means comprising an elongated floating electrode extending from another wall of said grid towards said hole and between said legs for a portion of its length and insulating means for supporting said floating electrode in proximity to said cathode.

5. The invention as described in claim 4 in which the portion of said floating electrode that extends between said legs is conical.

6. The invention as described in claim 4 in which said floating electrode extends through a hole in a wall of said grid opposite said one wall and is insulated from said opposite wall.

7. The invention as described in claim 4 in which the portion of said floating electrode that extends through said opposite wall is cylindrical and in which the portion of said floating electrode that extends between said legs is conical.

References Cited UNITED STATES PATENTS 3,141,993 7/1964 Hahn 315 DAVID J. GALVIN, Primary Examiner.

JAMES W. LAIVRENCE, Examiner.

V. LAFRANCHI, Assistant Examiner. 

